cmd/tsuru: app-remove should not guess the name of the app

[fsouza]

Copying the behaviour of heroku app:destroy, users should explicitly specify the application when running app-remove. We can still use the -a/--app flag, but

Current behavior:

% grep \"tsuru\" -A 1 .git/config
[remote "tsuru"]
    url = git@tsuru.mycompany.com:myapp.git
% tsuru app-remove -y superapp
App "myapp" successfully removed!

As you can see, I've accidentally removed "myapp", expecting "superapp" to be removed.